A silicon wafer (hereinafter may be simply referred to as a “wafer”) is required to have no crystal defects in an active layer of a device from the viewpoint of device characteristics. To satisfy this requirement, there has been used an annealed wafer manufactured by annealing a wafer at a high temperature to reduce crystal defects. The wafer is, however, apt to be easily contaminated by boron present in an environment (in peripheral air) to which the silicon wafer is exposed. For instance, boron is released as a compound such as an oxide (hereinafter may be referred to as “deposited boron” or simply as “boron”) from air filters used in a clean room, and the boron is deposited on a surface of a wafer left in the clean room.
When the wafer as described above is subjected to heat treatment with inert gases such as argon gas, boron deposited on the surface is not removed, and is diffused into the wafer to change resistivity in the vicinity of the wafer surface.
As a measure against boron present in an environment of a clean room, sometimes boron absorption filters or boronless filters are used for all air filters in the clean room, but it is necessary to frequently exchange the expensive filters with new ones, which results in a cost increase, and in addition even use of the filters as described above can not eliminate boron contamination completely.
On the other hand, when annealing is performed with hydrogen, boron dopant originally present inside the wafer easily outdiffuses, and further boron deposited on the surface evaporates or flies without diffusing into the wafer even by annealing, so that the boron concentration in the vicinity of the wafer surface decreases with the resistivity becoming problematically higher. To perform hydrogen treatment at a high temperature, a safeguard or the like is required to prevent explosion of hydrogen, which may raise such problems as a cost increase and a productivity decrease.
There is a method in which diluted hydrofluoric acid is used in the final stage of cleaning before annealing as proposed by the present applicant as a measure against boron contamination from such an environment (Japanese Patent Application No. 92155/2000). Although this method is extremely useful from the viewpoint of prevention of boron contamination, there is a problem with the method from the viewpoint of deposition of particles. Namely even if the final cleaning is performed with diluted hydrofluoric acid and then the wafer is cleaned with water, particles deposited in the diluted hydrofluoric acid are hardly cleaned down, and are carried up to the annealing step, so that, when the wafer is annealed, baking of the particles takes place to possibly cause a yield drop in fabrication of devices.
On the other hand, there has been proposed a method of reducing boron at an interface between an epitaxial layer and a bulk crystal in an epitaxial wafer (J. Robbins, A. J. Pidduck, J. L. Glasper, and I. M. Young, Appl. Phys. Lett. 55(12) 18 Sep. 1988). This method is characterized in that 100% hydrogen is used and the processing is performed in a depressurized state. Therefore, if it is tried to carry out the processing using an ordinary diffusion furnace, a safeguard for using hydrogen and facilities for performing the processing in a depressurized state are required, and therefore the method cannot be applied in a furnace not equipped with such facilities.
When an annealed wafer is manufactured in an atmosphere of hydrogen or inert gases represented by argon, generally a mirror-polished wafer to be annealed is prepared, contaminants such as heavy metals and organic materials deposited on the wafer surface are removed by wet cleaning and dried, and then the wafer is loaded into a heat treatment furnace.
When a mirror-polished silicon wafer is cleaned, a cleaning liquid (a chemical liquid) of various compositions is used, and a general cleaning method is of a proper combination of SC-1 (a liquid mixture of ammonia, hydrogen peroxide and water) cleaning, DHF (a diluted hydrofluoric acid aqueous solution) cleaning, and SC-2 (a liquid mixture of hydrogen chloride, hydrogen peroxide and water) cleaning, but when the DHF cleaning liquid is used as a cleaning liquid in the final stage of a sequence of cleaning steps, a natural oxide film on the wafer surface is removed with an active hydrophobic silicon surface being exposed, so that such troubles as deposition of particles or absorption of Cu, etc. easily occur.
Therefore, SC-1 or SC-2 is generally used in the final stage of the cleaning steps for the purpose to provide a hydrophilic wafer surface with a natural oxide film formed with the cleaning liquid thereon. This practice is also followed in wet cleaning performed before the wafer to be annealed is loaded into a heat treatment furnace, and after the wafer surface is finished to a hydrophilic one with a natural oxide film being formed thereon by cleaning, the wafer is loaded into a heat treatment furnace.
When the wafer is annealed in argon at a high temperature as described above, however, if boron has been deposited on the wafer before being subjected to annealing, the boron diffuses into the wafer by annealing with resistivity of the wafer surface being changed. When the resistivity of the wafer surface changes, in the case of, for instance, a MOS device, the on-off threshold voltage changes, which may not meet standard requirements. At the same time, various electric characteristics thereof also change.
In other words, when a wafer having boron deposited on a surface thereof is annealed in argon at a high temperature, crystal defects in the vicinity of the wafer surface are removed as in the case of annealing with hydrogen, so there is provided an advantage that the device characteristics are improved, but when the wafer surface is contaminated with boron, undesirable effects such as divergence of electric characteristics from the designed values occur, which may spoil a desirable improvement effect of the crystal quality in the vicinity of the wafer surface by annealing.